Ok, to hopefully answer a few questions ...
First up, it won't be possible to play a loyal servant of the Imperium. Sorry folks, but I just don't see how the two can gel together without someones character getting killed. The closest I could see would be a neutral, someone like a pirate or criminal perhaps, who isn't technically a follower of Chaos, but isn't going to seek to exterminatus them at first chance either. Something like a Dark Eldar might be doable, but not a Throne agent. Sorry.
What the game will be about ... that's a little more up in the air. I do have scenarios that I can use, and indeed intend to use to start things up. If the players want to drive the game forwards, I can probably accommodate that too. We need minions .. let's take over the pirate vessel 'Atterete Dominatum'. I want to construct a daemon weapon .. cool, let's find the true name of a daemon, forge a suitable blade, research the binding ritual, then slaughter 1000 slaves to bring the favor of the Gods. If the PCs have goals they want to achieve, those will be the adventures.
To start with, since this is something of an experiment, I plan to use the FFG intro adventure Broken Chains. If it works, we go on from there, and if we play long enough, yes, a Black Crusade could be the eventual campaign goal.
In terms of characters, think about the 40k background, lore and fiction, and the assorted followers of Chaos. Chaos Space Marines, veterans of the Long War, those who fought alongside the Warmaster and scorched the Imperial Palace. Renegade marines of a more recent vintage. Rogue Traders turned pirate warlord. Psychers. Cult members and leaders. Heretek Dark Mechanicus. Traitor Guard. Daemonhosts. A fallen Inquisitor. There are a myriad of possibilities.
Chaos Space Marines make wonderful armour plated killing machines, but they may not be quite so useful at deciphering that book of forbidden rituals, or infiltrating the Hive Spire in establish a cult. There can be roles for everyone, and I try to tailor my games so everyone gets their moment in the spotlight.
Now, in terms of rules, as I said it's going to be mostly freeform. I will be swiping a few ideas from games to add what I hope will be guidelines and a cinematic feel.
First up, each character will have a number of Aspects, swiped from the FATE systems (for those who know them). These are there partially to help flesh out and define the characters.
For example, my Night Lords space marine could have the following aspects.Son of the sunless world
- As a Night Lords marine, he can see in the dark far better than most. He has a talent for stealth, ambush and terror tactics. It also means he will favor a slow approach, to give time for terror attacks (preferably against helpless targets) to build an atmosphere.Veteran of the Long War
- He has fought against the Imperium for thousands of years. He has been many places, and made many friends .. and just as many enemies.
Aspects could be people you know, gear, contacts, personality traits, anything that could be used to give the character life, and generate adventure.
And before anyone asks, your own Repulsive Class Grand Cruiser is not an acceptable aspect.
Knowing the location of an abandoned and almost intact one does make for a great story seed though. It would make for a wonderful base of operations. If it's still there. And you can find enough resources to reactivate it. And someone else doesn't decide it will make an even better base for them
Aspects are most useful when they're a two edged sword, something that can both give you a benefit, or cause you problems. When I, as the GM, get to use them to cause you problems, you get a Plot Point
Plot points can be used for the cinematic moments. Using one gives you a boost for whatever you want to do. Make that impossible shot. Stumble across the hidden latch that opens the secret door when you bang your hand on the wall in frustration. Discover that the that mysterious pirate you need to sway to your cause is actually someone you know. Mother? Is that you?
You get the idea. If you use them in conjunction with an aspect, you'll get a further bonus.
As it's mostly freeform though, we want to go with the rule of cool. If it advances the story, and you're not being an absolute Munchkin, it's good. The Chaos Marine with the heavy bolter ought to be able to gun down an entire squad of Guardsmen with a sustained burst. If he is trying to take out the Lightning fighter bearing down on the group with his last remaining missile, well, that's the point we want to make a roll.
And maybe consider the spending of a plot point too :)
The Eye of the Gods (my trusty d8 here) will determine how well you do.
1-2: Wrath of the Gods - Things ... don't work out so well
3-4: The Gods gaze is elsewhere .. neutral result. Work with it.
5-7: You please the Gods ... what you try works
8: Favour of the Gods ... you're rewarded with an exceptionally good result.
Trying something that plays to your patron power will get a +1 (the warrior of Khorne engaging a dreadnought with a meltabomb). Trying something that would offend the power gets you a -1 (that same warrior of Khorne trying to talk their way out of a fight). If a post is exceptionally well written, that may gain a +1 at my discretion. The Chaos Gods can be fickle that way
Now, aside from me sending trouble your way with the aspects, the other way to earn plot points is to impress the hell out of the group. An extremely well written post. A line that cracks everyone up. Something that makes the other people go "Ok, that was cool." They nominate you for a plot point. If it's judged worthy, you get the plot point.
I don't plan on being too miserly with these, as they can be used to provide those cinematic moments, but at the same time I wouldn't give them out like candy either. Using them a few times in each scenario is probably a good rule of thumb. Think of the 40k fiction you've read. It has cool moments, but not every other page.
So ... does that help any?